Do you want credits for your images and/or you brand then you should consider adding a watermark to all images you are posting online. With your name or website added as a watermark to a photo you are more likely to get the credits. The harder you make the watermark to remove the more sure you can be of getting the credits.

Another use of a watermark could be to stamp the file date/time the photo was taken to the image to make sure the image has not been tampered with.